We intend to continue the development of inbred lines of rabbits of known origin and distinct genetic characteristics. As a rule, rabbit inbreeding at the Jackson Laboratory proceeds through continuous sib-mating. When a line which originates from a hybrid population is maintained through inbreeding and a number of matings at each generation, the line tends to split and to form parallel individual families. As genetic fixation proceeds it is possible that, after many generations of inbreeding, a family of one line may differ from another within the same line at a single locus for a biological system involving numerous loci (for instance, the histocompatibility system). Such families would, for all practical purposes, constitute essentially congenic lines. We test for single-locus differences through skin grafts between members of the same or of different families at different generations, all however within the same line. This testing procedure serves to measure the actual advance in genetic fixation which arises through inbreeding; simultaneously, it can detect mutations or residual heterozygosity, either of which could inaugurate new congenic lines. In our project, we are maintaining the most advanced inbred lines of rabbits. It is hoped that the establishment of such lines, having a defined origin, genetic markers, and specific immunological and biological characteristics, will prove useful for studies on histocompatibility as well as on other biological systems.